FCP Codec not found "Help!"

[Francis South]

Hi, I edited a short a few days ago in the computer lab at school using FCP and saved everything to my external hard drive . I now want to make some tweaks to the short at home. My iMac has FCP 6 and I've done projects before on it but now when I opened my short it gives me an error "Codec not found. You may be using a compression type without the corresponding hardware card" then after clicking OK ten times it gives me another error saying "Out of memory" and gives me that error if I open any sequence of my project. I am able to create new sequences however and it worked fine when I tried loading it to a friend's Macbook. I tired making my external drive (which has 450 gigs of space left) its scratch but that did not work. The footage I'm working with is in Apple Pro Res 422, 1920 x 1080p, 23.98 fps and I have 4 GB of RAM so it should work like it has worked before, so what's going on why cant my computer open my project?